The present invention relates to thermal transfer means, and more particularly to thermal transfer means used in conjunction with electronic systems.
It is standard practice in electronic systems to utilize as thermal transfer means a conductive cooling thermal plane with printed circuit boards upon which are mounted heat generating electronic components and to use other thermal transfer means with rf modules and electronic chassis. Such thermal plane cooling makes use of aluminum or copper members mated to one or more printed circuit boards, with the thermal plane being interfaced with a heat sink at the edges of the thermal plane. A thermal transfer means as seen in U.S. Pat. No. 4,602,678, comprises a silicone layer with embedded thermally conductive metal oxides. In copending application Ser. No. 878,102 (W.E. Docket Number 52,938) entitled "High Density Microelectronic Packaging Module For High Speed Chips" owned by the assignee of the present invention, an aluminum thermal plane is used for cooling printed circuit board attached to opposed sides of the thermal plane in a Standard Electronic Module.
For a variety of electronic applications, such as airborne or aerospace applications, it is highly desirable to minimize the weight of the thermal transfer means. In particular, the aluminum thermal plane typically utilized may account for a not insignificant portion of the weight of the electronic package or system. It is therefore highly desirable to provide a lightweight, high efficiency heat transfer thermal plane for use with electronic systems.
A recent innovation in aerospace systems is the use of fiber reinforced composites for a variety of applications because of the high strength, lightweight character of such composites. A thermally conductive printed wiring board laminate is taught by U.S. Pat. No. 4,609,586. The laminate combines a low coefficient of thermal expansion in the X-Y direction with thermal conductivity. The laminate comprises a support fabricated from a graphite reinforced matrix material adhesively secured to a printed wiring board of dielectric glass fiber reinforced resin. The graphite fibers are cross-plied in adjacent layers to provide the desired low coefficient of thermal expansion in the X-Y direction.
As mentiond above with respect to the copending application, conductive cooling of circuit board pairs, using what may be called a "thermal plane", is being emphasized for future electronic systems. In particular, conductive cooling using a thermal plane is being implemented in the Standard Electronic Module (SEM), which is a military standard configuration of two circuit boards which will be used in large quantities in various future systems.
In the SEM, as described in the aforementioned copending application, the circuit boards are permanently attached to the thermal plane using a conductive epoxy adhesive. The thermal plane is in turn non-permanently fixed within the chassis by wedgelocks along two side edges, and by a connector along the bottom edge.
From the point of view of the thermal plane, the circuit boards attached to it are weighted epoxy/glass laminates which generate heat. Two functions are provided by the thermal plane to the pair of boards. First, the thermal plane supports the circuit boards within the chassis. For this function, the thermal plane is required to possess sufficient strength to support the circuit boards during all anticipated situations of vibration and shock. The second function of the thermal plane is, as already mentioned, to provide a conductive path for the heat generated by the circuit board components. Heat flows into the thermal plane from the components mounted on the circuit boards. Heat then flows along the thermal plane, in opposite directions, to the thermal plane edges adjoining the heat sink walls. Finally, heat flows out of the thermal plane and passes into the tabs on the heat sink walls. Wedgelocks exert pressure on the thermal plane edges and the heat sink tabs to assure good thermal contact between the thermal plane and the heat sink tabs.
There is a significant difference in the size of the areas over which heat flows into and out of the thermal plane. Heat flow into the thermal plane is spread out over the relatively large side areas of the thermal plane. In comparison, the outward flow is concentrated over the relatively small contact areas between the thermal plane and the heat sink tabs. Heat flux (heat flow/area) is comparatively low into, and high out of, the thermal plane.
Overall, the weight of the thermal plane is to be minimized. It is known that of the two functions the thermal plane is required to perform, namely cooling and structural support of the attached circuit boards, cooling governs the thermal plane design. In other words, a thermal plane thick enough to have an acceptable low thermal resistance undoubtedly has adequate strength. In order to minimize weight, then, the material thermal conductivity should be maximized and the material density should be minimized. More concisely, the specific thermal conductivity (conductivity/density) should be maximized.
A class of graphite fibers has been developed which has very high thermal conductivity along the length of the fibers. Fibers in this special class are distinguished from typical graphite fibers in that the thermal conductivity of this class of fibers exceeds the conductivity of many metals. Whereas, the thermal conductivity of most graphite fibers is significantly less than that of metals. These high conductivity graphite fibers are of the "Pitch" type rather than the "PAN" type, and are manufactured by the Amoco Advanced Composites Corporation. Two types of conductive graphite fibers, designated P100 and P120, are of particular interest. Both fiber types have thermal conductivities along the length of the fiber greater than that of aluminum, with P120 being slightly higher than P100.
Graphite fibers are not used alone, but are mixed with a matrix material to create composite materials which possess structural characteristics. Composite materials formed of graphite fibers and a matrix typically possess high stiffness and strength. The matrix material used with graphite fibers is often an epoxy because of the good properties of the composite that result. For the conductive graphite fibers P100 and P120, the matrix material most used is an epoxy. Other matrix materials which can be used are metals, such as aluminum, and organic resins which are bondable to the fibers.
The baseline material for the prior art thermal plane can be considered to be aluminum. In addition to being the most common metal used in aircraft and aircraft electronic systems, aluminum also has a high specific conductivity compared to other metals. The specific conductivities of the new conductive graphite fiber composites, however, are much superior to that of aluminum. A complete comparison is presented in Table 1.
TABLE 1 ______________________________________ Comparison of Conductive Composites and Aluminum Pl00/ P120/ 6061 ALU- EPOXY EPOXY MINUM ______________________________________ DENSITY 0.065 0.065 0.10 (lb./in) (112 lb/ft.sup.3) (112 lb/ft.sup.3) (173 lb/ft.sup.3) THERMAL 180. longi- 222. longi- 100. CONDUCTIVITY tudinal tudinal (BTU ft/hr-ft.sup.2 -.degree.F.) 0.5 trans- 0.5 trans- verse verse SPECIFIC 1.61 longi- 1.98 longi- 0.578 THERMAL tudinal tudinal CONDUCTIVITY 0.0045 trans- 0.0045 trans- (BTU-ft.sup.2 /lb-hr-.degree.F.) verse verse ______________________________________
The material properties presented in Table 1 for the composites are based on a specific volume percentage of fibers and epoxy. It can be appreciated that since the properties of the constituent materials (fiber, matrix) are different, the properties of the mixture are dependent on the relative amounts of the constituents present. The data is for a 60 percent fiber/40 percent epoxy mixture, which is the optimum mixture from the points of view of processing factors and cured material properties.
Considering the values presented in Table 1, it can be seen that P100 composite is 2.79 times better at conducting heat, per unit weight, than aluminum (1.61/0.578=2.79). More specifically, P100 composite is 2.79 times better than aluminum when heat is conducted along the fibers (longitudinal direction). Perpendicular to the fibers (transverse direction), P100 composite actually conducts heat much poorer than aluminum. This fact can be understood when it is considered that the thermal conductivity of epoxy is very low (approximately 0.2 BTU-ft/hr-ft.sup.2 -.degree. F.) and also the transverse conductivity of the fiber is very low, so that for heat to conduct through the composite perpendicular to the fibers, it must alternately pass through fibers transversely and the epoxy matrix between the fibers. Conversely, for heat conduction through the composite parallel to the fibers, virtually no heat passes through the epoxy, but rather it passes down the fibers themselves. This assumes that long continuous fibers rather than short "chopped" fibers are used. Any application of conductive graphite fiber composites for heat transfer must account for the poor transverse conductivities of the materials. Nevertheless, if conduction along the fibers is the means by which heat is carried, conductive composites offer great weight advantages compared to metals.
The total weight saved in a system by the use of conductive composite thermal planes instead of aluminum thermal planes can be appreciated by a hypothetical example. Assuming that the typical aluminum thermal plane is 5".times.6".times.0.080" and that 1000 thermal planes are used in a system, the total weight of aluminum thermal planes would be: EQU WT.sub.AL =5.0.times.6.0.times.0.080.times.0.10.times.1000=240 lbs.
Assuming that P100 graphite/epoxy is only 2.0 times better than aluminum at conducting heat per unit weight (rather than 2.79) the weight of the P100 thermal planes would be 240/2=120 lb., one-half the weight of aluminum thermal planes.